Project Summary Basic and clinical research that was initiated in the late 20th century and continues to this day has transformed our thinking about brain resilience. The term brain plasticity is often invoked to describe adaptive changes in the adult brain. This proposed research program concerns the study of neuroplasticity in a seasonal context. This phenomenon involves naturally occurring brain changes that allow an animal to cope with seasonal variation in the physical and social environment to facilitate survival and reproductive success. We study how the steroid hormone testosterone regulates brain changes and the associated changes in behavior in seasonally breeding male and female canaries. These birds possess a well-defined neural circuit, called the song system, which regulates an important learned social behavior, song. Song varies seasonally in that it is more common and more complex in spring when it is produced primarily by males to attract females. Females also produce song but at a lower rate with simpler structure. Key forebrain song nuclei are much larger in volume in the spring in males than in the fall when song is produced less frequently; these song nuclei volumes also are larger in males than in females. Such seasonal changes in brain and behavior may recapitulate behavioral and brain plasticity that occurs during the vocal learning process. Our recent work in canaries has shown that testosterone acts in a pleiotropic manner to regulate multiple aspects of song behavior by having anatomically specific effects in the forebrain song nuclei and in the medial preoptic area, that testosterone can induce male-like changes in neurogenesis and in song behavior in adult female canaries but apparently cannot induce a complete sex reversal, and finally that species variation in perineuronal nets (specialized structures of the extracellular matrix) correlates with species variation in adult vocal plasticity. In this proposal, we capitalize on these findings by investigating the regulation by steroid hormones of brain plasticity and the related song learning and behavior as well as sex differences in their action. The proposal is organized into four aims. In Aim I we ask whether steroid hormone-induced plasticity in the song control nucleus is similar in males and females. Aim II investigates indirect effects of testosterone on song behavior and brain activity related to song by assessing how steroid action in the medial preoptic nucleus indirectly facilitates song-related immediate early gene expression and singing behavior in the song control circuit. Aim III addresses the critical question of the function of adult neurogenesis in songbirds as we will ablate neuroblasts in a site-specific manner with the use of the focused X- ray irradiation method in the ventricular zone in testosterone-treated male and female canaries and assess the morphological and behavioral consequences of these ablations. In Aim IV we will test the hypothesis that perineuronal nets play an important role in limiting brain and behavioral plasticity as it applies to song learning. This research will contribute to the development of more mechanistic insights into brain plasticity as well as help us identify how steroid hormones might serve in a neuroprotective and/or cognitive enhancer-like manner.